Multilevel Ferroelectric Memory Cell for an Integrated Circuit

ABSTRACT

An integrated circuit includes a ferroelectric memory cell. The ferroelectric memory cell includes a ferroelectric layer stack comprising at least one ferroelectric material oxide layer. Each of the ferroelectric material oxide layers includes a ferroelectric material that is at least partially in a ferroelectric state. The ferroelectric layer stack comprises at least two ferroelectric domains. Further, the voltage which is to applied to the layer stack to induce polarization reversal differs for the individual domains such that polarization reversal of individual domains or of a portion of the totality of ferroelectric domains within the ferroelectric material of can be attained.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.15/367,629 filed Dec. 2, 2016, which claims foreign priority under 35U.S.C. § 119(a)-(d) to Application No. DE 102015015854.6 filed on Dec.3, 2015, the entire contents of which are hereby incorporated byreference.

BACKGROUND

Many electronic devices and systems have the capability to store andretrieve information in a memory structure. A number of different memorystructures are used in such systems. One prominent volatile memory is aDRAM structure that allows for high speed and high capacity datastorage. Some examples of non-volatile memory structures include ROM,Flash structures, ferroelectric structures (e.g., FeRAM and FeFETdevices) and MRAM structures.

With regard to ferroelectric (FE) structures, these structures can be inthe form of a capacitor (e.g., a FeRAM) or a transistor (FeFET), whereinformation can be stored as a certain polarization state of theferroelectric material within the structure. The ferroelectric materialthat can be used is hafnium dioxide or zirconium dioxide or a solidsolution of both transition metal oxides. In the case of pure hafniumoxide, the remnant polarization can be improved by a certain amount ofdopant species which has to be incorporated into the HfO₂ layer duringthe deposition.

The ferroelectric material is intended to partially or fully replace thegate oxide of a transistor or the dielectric of a capacitor. Theswitching is caused by applying an electrical field via voltage betweentransistor gate and transistor channel. Specially, for n-channeltransistors, ferroelectric switching after application of a sufficientlyhigh positive voltage pulse causes a shift of the threshold voltage tolower or negative threshold voltage values. For p-channel transistors anegative voltage pulse causes a shift of the threshold voltage to morepositive threshold voltage values.

FeFET memory enjoys a number of advantages over other Flash storagedevices. It generally offers faster read and write access times andlower power consumption during write operation due to the differentphysical storage mechanism. Further, it is comparatively easy tointegrate into High-k metal gate CMOS technology. These advantages, andothers, may explain the increasing popularity of FeFET memory forembedded storage as well as for stand-alone applications to be adoptedin devices such as memory cards, USB flash drives, mobile phones,digital cameras, mass storage devices, MP3 players and the like.

The areal bit-density of a memory is determined by three parameters: thememory cell size, the memory array efficiency that is the ratio betweenmemory array area and overall chip area including the driving circuitry,and the number of bits that are stored within each of the memory cells.In contrast to single-level cell (SLC) memory, which can store only onebit per cell, multi-level cell (MLC) memory has the ability to storemore than one bit of data per cell. In an MLC Flash cell, the data istypically stored in the form of 4 or 8 distinguishable threshold voltagelevels, thus yielding two or three bits per cell.

SUMMARY

In accordance with embodiments described herein, an integrated circuitcomprises a ferroelectric memory cell. The ferroelectric memory cellcomprises a ferroelectric layer stack. The layer stack comprises aferroelectric material that is at least partially in a ferroelectricstate. The ferroelectric material further comprises multipleferroelectric domains with different coercive voltages. Therefore, thevoltage that has to be applied to induce polarization reversal in afirst portion out of the totality of all domains is different from thevoltage that has to be applied to induce polarization reversal in asecond portion out of the totality of all domains. In that way, morethan two polarization states can be induced in the ferroelectric layerstack.

Second option: The ferroelectric memory cell comprises a layer stack,comprising a first ferroelectric material oxide layer and at least onesecond ferroelectric material oxide layer. Each of the oxide layerscomprises a ferroelectric material that is at least partially in aferroelectric state. The first ferroelectric material oxide layer andsecond ferroelectric material oxide layer are arranged in a multi-layerstructure and can be deposited directly on top of each other or can beseparated by a third layer which can be a conducting or insulatingmaterial. Further, the ferroelectric material of the first ferroelectricmaterial oxide layer comprises a coercive voltage that is different fromthe coercive voltage of the ferroelectric material oxide layer. In thecase of multiple ferroelectric material oxide layers, each of the layerscomprises a coercive voltage that is different from the coercive voltageof the other ferroelectric storage layers.

Third option: The ferroelectric memory cell comprises a layer stack,comprising a first ferroelectric material oxide layer and a secondferroelectric material oxide layer. Each of the layers comprises aferroelectric material that is at least partially in a ferroelectric.The first ferroelectric material oxide layer and second ferroelectricmaterial oxide layer are arranged in a multi-layer structure and areseparated by a third layer that is a conducting material. Further, afterstructuring both ferroelectric layers, the capacitive voltage dividerbetween the two storage layers is balanced in a way that the twoferroelectric material oxide layer undergo ferroelectric polarizationswitching at different voltages applied to the layer stack structure.

In accordance with other embodiments described herein, a method forprogramming a ferroelectric memory cell, where the ferroelectric memorycomprises a ferroelectric memory cell and a voltage source, comprising aferroelectric layer stack, to apply a voltage to the ferroelectric layerstack, comprises applying a write voltage pulse sequence to theferroelectric layer stack via a voltage source. The write voltage pulsesequence comprises applying a pulse with an amplitude equal or greaterthan the coercive voltage having a value of UC that is suitable tochange a polarity of at least a first portion of domains of theferroelectric layer stack, whereas a second portion of domains can stayunaffected, and UC is the coercive voltage needed to switch polarizationof the first portion of domains into the opposite direction.

Second option: A method for programming a ferroelectric memory cell,where the ferroelectric memory comprises a ferroelectric memory cell anda voltage source, comprising a first and at least one secondferroelectric material oxide layer, to apply a voltage to theferroelectric memory cell, comprises applying a write voltage pulsesequence to the ferroelectric layer stack via the voltage source. Thewrite voltage pulse sequence comprises applying a pulse with anamplitude equal to or greater than the coercive voltage having a valueof UC that is suitable to change a polarity of the first ferroelectricmaterial oxide layer, whereas the polarity of the second ferroelectricmaterial oxide layer stays unaffected, and UC is the coercive voltageneeded to switch polarization of the ferroelectric material oxide layerinto the opposite direction. Further, the write voltage pulse sequencecomprises applying a pulse with an amplitude equal to or greater thanthe coercive voltage having a value of U_(Cmax) that is suitable tochange the polarity of all domains of the ferroelectric layer stack.

The above and still further features and advantages of embodiments ofthe present invention will become apparent upon consideration of thefollowing detailed description thereof, particularly when taken inconjunction with the accompanying drawings wherein like referencenumerals in the various figures are utilized to designate likecomponents.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1a depicts a cross-sectional view of an example embodiment of ametal ferroelectric semiconductor (MFS) structure, where theferroelectric layer comprises multiple domains.

FIG. 1b depicts a cross-sectional view of an example embodiment of ametal ferroelectric metal (MFM) structure, where the ferroelectric layercomprises multiple domains.

FIG. 1c depicts a cross-sectional view of an example embodiment of ametal ferroelectric semiconductor (MFS) structure, representing a FeFETstructure where the gate, source, drain and bulk terminals are connectedto voltage sources, applying the voltages VG, VS, VD and VB,respectively.

FIG. 1d depicts a cross-sectional view of an example embodiment of ametal ferroelectric metal (MFM) structure, representing a storagecapacitor of a FRAM memory cell, where the terminals are connected tothe BL and an access transistor, respectively. The access transistor isconnected to a WL and a SL within a memory array.

FIG. 2a depicts a cross-sectional view of an example embodiment of ametal ferroelectric semiconductor (MFS) structure, with twoferroelectric layers stacked one above the other.

FIG. 2b depicts a cross-sectional view of an example embodiment of ametal ferroelectric metal (MFM) structure, with two ferroelectric layersstacked one above the other.

FIG. 3a depicts a cross-sectional view of a cell structure, with twoferroelectric layers of different area, embedded within the gate stackand separated by a floating conductive layer.

FIG. 3b depicts a cross-sectional view of a MFMFM structure, where twoferroelectric layers of different lengths are separated by a floatingconductive layer.

FIG. 3c depicts a cross-sectional view of a MFMFM structure, where theferroelectric layers together with a conductive interlayer form acapacitive voltage divider.

FIG. 4 depicts a cross-sectional view of a cell structure, comprisingtwo MFS structures having different gate lengths. The two gates areconnected to the same electrode.

FIG. 5 depicts a cross-sectional view of a 3D-cell structure, comprisinga multitude of gate layers which are isolated by isolation layers,comprising transistor channel pillars comprising a semiconductingmaterial, and comprising a ferroelectric layer stack surrounding thetransistor channel pillars.

FIG. 6a depicts the operating conditions to write and read amulti-domain ferroelectric memory cell.

FIG. 6b depicts a state diagram to operate a two-domain ferroelectriccell structure.

DETAILED DESCRIPTION

In accordance with embodiments described herein, a ferroelectric memorycell of an integrated circuit comprises a structure including aferroelectric material comprising a plurality of ferroelectric domains.The individual ferroelectric domains comprise different coercivevoltages such that by applying a voltage larger than the minimumcoercive voltage but smaller than the maximum coercive voltage of alldomains to the material, a first portion of domains out of the pluralityof all domains can be switched.

The ferroelectric memory cell structure can be formed, e.g., as a metalferroelectric semiconductor (MFS) structure (e.g., for FeFET structures)as depicted in FIG. 1a . The ferroelectric memory cell can also beformed as a metal ferroelectric metal (MFM) structure (e.g., for FeRAMstructures) as depicted in the cross-sectional view of FIG. 1 b.

Referring to the cross-sectional view of FIG. 1a , an example embodimentof a MFS structure 100 includes a support structure 101 comprising acarrier material, such as a silicon compound. An interfacial layer 105can be formed over layer 101, which can comprise any suitablematerial(s) including, without limitation, SiO₂ or SiON or any othersilicon containing material. A ferroelectric material oxide layer 103 isformed over layer 105 or directly over layer 101. The ferroelectricmaterial oxide layer comprises different ferroelectric domains 103 a,103 b and 103 c, which can coincide with different grains in apolycrystalline film. Each of the domains can comprise a differentcoercive voltage, originating from a different crystal orientation ofthe individual grain, a different internal stress, a different surfaceenergy due to different grain sizes or different doping concentration,or other differences in the physical or chemical composition. The term“ferroelectric material,” as used herein, refers to a material that isat least partially in a ferroelectric state. For example, theferroelectric material may comprise any of HfO₂, ZrO₂, any ratio of Hfand Zr combined with oxygen (e.g., Zr_(x)Hf_(1-x)O₂, where x<1) as wellas any combinations thereof.

The ferroelectric material oxide layer 103—and if existing—together withthe interfacial layer 105 forms a layer stack 109. In one embodimentdescribed herein the layer stack 109 represents a storage layer of amemory device.

The conductive layer 106 can comprise any one or more suitableconductive metals including, without limitation, Ti, TiN, TiSi, TiAlN,TaN, TaCN, TaSi, W, WSi, WN, Ru, RuO, Re, Pt, Ir, IrO, Ti, Ni, NiSi, Nb,Ga, GaN, C, Ge, Si, SiC oder GeSi. It is to be understood, that thematerial of the conductive layer can be chosen such that thework-function of the respective material influences the coercive voltageof the adjacent ferroelectric material oxide layer in a manner that isbeneficial for the operation of the ferroelectric memory cell.

Referring to the cross-sectional view of FIG. 1b , another exampleembodiment of a MFM structure 110 includes two conductive structures 106and 108, where the conductive layer can be formed of conductivematerials such as described herein in relation to layer 106. Aferroelectric layer stack 109 is formed to be sandwiched between the twoconductive structures 106 and 108. The layer stack 109 comprises aferroelectric material oxide layer 103 which is formed in between thetwo conductive structures. The ferroelectric material comprisesdifferent ferroelectric domains 103 a-c, which can coincide withdifferent grains in a polycrystalline film. Each of the domains cancomprise a different coercive voltage, originating for example, but notlimited to, from a different crystal orientation of the individualgrain, a different internal stress, a different surface energy due todifferent grain sizes or different doping concentration.

An example process for forming the MFS structure of FIG. 1a is describedas follows. A carrier structure 101 is provided that may comprise a Si(silicon) compound, such as SiGe (silicon-germanium) orsilicon-on-insulator (SOI). It is noted that other semiconductormaterials can also be provided as the carrier structure 101 including,e.g., III-V semiconductor compounds such as GaAs or any other suitablesubstrate material. The carrier structure 101 may have already beenprocessed so as to include components and/or other devices already beformed within the carrier structure.

An interfacial layer 105 can be formed on the support structure 101 toimprove the transistor channel to gate oxide interface quality, toreduce the number of charge traps or to prevent chemical reactionsbetween the support structure 101 and the ferroelectric material oxidelayer 103.

Referring to the cross-sectional view of FIG. 1b , in another exampleembodiment a conductive layer 108 can be formed on a supportingstructure. The conductive layer 108 can comprise any one or moresuitable conductive metals including, without limitation, Ti, TiN, TiSi,TiAlN, TaN, TaCN, TaSi, W, WSi, WN, Ru, RuO, Re, Pt, Ir, IrO, Ti, Ni,NiSi, Nb, Ga, GaN, C, Ge, Si, SiC oder GeSi. The conductive layer 108can be formed utilizing any one of atomic layer deposition (ALD), metalorganic atomic layer deposition (MOALD), chemical vapor deposition(CVD), metal organic chemical vapor deposition (MOCVD), physical vapordeposition (PVD), molecular beam epitaxy (MBE) deposition, Sol-gel orany other suitable deposition technique that facilitates formation ofthe layer. The thickness of conductive layer 108 can be in the range of0.5 nm to 10 nm or 1 nm to 300 nm.

The ferroelectric material oxide layer 103 is formed on the interfaciallayer 105 or directly on the support structure 101 for the MFS structure100 of FIG. 1a or is formed on the conductive layer 108 for the MFMstructure 110 of FIG. 1b . The ferroelectric layer 103 and theinterfacial layer, if available, form the layer stack 109. In eachembodiment, layer 103 can be formed utilizing any one of atomic layerdeposition (ALD), metal organic atomic layer deposition (MOALD),chemical vapor deposition (CVD), metal organic chemical vapor deposition(MOCVD), physical vapor deposition (PVD), molecular beam epitaxy (MBE)deposition, Sol-gel or any other suitable deposition technique thatfacilitates formation of the layer including the ferroelectric materialas described herein (i.e., oxygen and at least one of Hf and Zr), wheregrowth of the layer should be poly-crystalline. Any suitable number andtypes of precursors may be utilized to introduce elements such as Hf andZr into the layer 103 utilizing any of the deposition techniques asdescribed herein. The layer 103 is formed to have a suitable thickness,e.g., in the range from about 2 nm to 500 nm. In an example embodiment,the thickness range from layer 103 can be within the range from about 2nm to about 15 nm.

In addition, the ferroelectric material oxide layer 103 can be formed toinclude, in addition to the ferroelectric material, dopants or furtheradditives that may support the crystallization of the layer 103 into astate having ferroelectric properties. The additives can be includedwith the precursor materials, e.g., so as to be included duringformation of the layer 103. Alternatively, the additives can beintroduced into the formed layer 103 by ion implantation or any othersuitable process. A concentration of the further additives within thelayer may be set within a range from about 0.05 at % (atomic percent, asmeasured by ratio of additive atoms to ferroelectric material atoms) toabout 30 at %, within a range from about 0.05 at % to about 10 at %,within a range from about 0.05 at % to about 5 at %, within a range fromabout 0.5 at % to about 3.5 at %, or a range from about 1 at % to about3.5 at %. In general, the amount of the further additives may depend onthe thickness of the layer 103. When increasing the thickness of thelayer 103, the concentration of the further additives may also have tobe increased to achieve a desired crystallization having ferroelectricproperties.

Any suitable additives may be provided within the ferroelectric materialoxide layer 103 including, without limitation, any one or more of C, Si,Al, Ge, Sn, Sr, Mg, Ca, Sr, Ba, Ti, Zr, Hf, Gd, Sc, La (e.g., providingZr as an additive in a HfO2 layer), Ti, and any one or more of the rareearth elements (e.g., Y, Gd, etc.). In particular, it has beendetermined that certain additives having an atomic radius that is aboutthe same as or greater than Hf are particularly suitable as dopants foroptimizing ferroelectric (FE) properties of the ferroelectric materialoxide layer 103 when utilizing Hf in the layer. In contrast, certainadditives having an atomic radius smaller than Hf can causeanti-ferroelectric (anti-FE) properties at phase boundaries between themonoclinic and tetragonal/cubic phases of HfO₂. It has further beendetermined that additives having an atomic radius about the same as orgreater than Hf can be doped at larger ranges of concentrations withinthe ferroelectric material oxide layer in relation to other additiveswhile still supporting FE properties of the ferroelectric material oxidelayer. Other additives having the same valence as Hf can also bebeneficial as dopants to reduce charge trapping characteristics of theferroelectric material oxide layer by reducing open bonds within theHfO₂ host lattice of the layer.

The conductive layer 106 for the embodiments of FIG. 1a and FIG. 1b canbe formed utilizing any one of atomic layer deposition (ALD), metalorganic atomic layer deposition (MOALD), chemical vapor deposition(CVD), metal organic chemical vapor deposition (MOCVD), physical vapordeposition (PVD), molecular beam epitaxy (MBE) deposition, Sol-gel orany other suitable deposition technique that facilitates formation ofthe layer. The conductive layer can be formed from any of the types ofconductive materials as previously described for conductive layer 108,and the thickness of conductive layer 106 can be in the range of 1 nm to10 nm or 1 nm to 300 nm.

The conductive layer 106 for the embodiments of FIGS. 1a and 1b providesa covering layer for the ferroelectric material oxide layer 103 and canalso serve as an electrode for the implementation of the MFS (e.g., gateelectrode) or MFM within a semiconductor memory cell structure.

In an alternative embodiment, a further covering layer can also beprovided between the ferroelectric material oxide layer 103 andconductive layer 106. The further covering layer can be deposited priorto forming the conductive layer 106 utilizing any suitable depositionprocess such as any of the previously described processes, and thefurther covering layer can comprise any suitable materials such as SiO₂,Al₂O₃, Sc₂O₃, Y₂O₃, BaO, MgO, SrO, Ta_(x)O_(y), Nb_(x)O_(y), TiO, andlanthanum dioxides. The further covering layer may also be formed in asame deposition process with the second ferroelectric material oxidelayer 103 by changing the supply of source/precursor materials duringthe deposition process (thus allowing the deposition to be performedwithin the same deposition chamber). For example, when forming theferroelectric material oxide layer 103 from HfO₂, a precursor gasincluding oxygen may be continuously provided during deposition of theamorphous layer and the further covering layer. Hafnium precursor gasmay be included with the oxygen precursor gas first in the depositionchamber to form the layer 103, where the hafnium precursor gas flow isthen switched to a precursor gas flow (e.g., Si) to with the continuingflow of oxygen to form the further covering layer (e.g., SiO₂).

After the layers have been formed, an anneal process is carried out atone or more suitable temperatures and for one or more suitable timeperiods to achieve a suitable amount of crystallization for theferroelectric material within the ferroelectric material oxide layer103. In particular, the anneal process is carried out to heat theferroelectric material oxide layer 103 to a temperature that is abovethe crystallization temperature of the ferroelectric material so as toat least partially alter its crystal state from amorphous tocrystalline, thus resulting in a crystallized oxide within theferroelectric material oxide layer 103. A crystallization temperaturemay be chosen in a range of, e.g., from about 400° C. to about 1,200° C.depending on the thermal budget of the used devices. A preferredcrystalline temperature for the annealing process is at a temperaturethat is above the onset of crystallization for the ferroelectricmaterial (when the ferroelectric material is amorphous, i.e., after thelayer 103 is deposited and before annealing occurs) and is furthergreater than about 500° C., or at a temperature that is above the onsetof crystallization for the ferroelectric material and is further greaterthan about 300° C. The time period for annealing can be from about 0.01second to about 12 hours. These annealing temperature ranges inducepartial crystallization of the ferroelectric material oxide layer 103(e.g., crystallization to a suitable level within the layer 103 that isless than complete crystallization of the ferroelectric material).

The partial crystallization of the ferroelectric material oxide layer103 results in ferroelectric domains within the layer 103 that are in aferroelectric state (e.g., at least partially). The crystallized layer103 may exhibit different dipole moments. The orientation of the dipolemoments in the crystallized layers 103 can result in a variation of thecoercive voltage of the individual domains. By applying suitablevoltages to the layer stack 109 the dipole orientation can be switchedfor a portion of ferroelectric domains with a coercive voltage that islower than the applied voltage, while other domains stay unaffected andno change of the dipole orientation is induced for domains with acoercive voltage that is greater than the applied voltage. In otherwords, the ferroelectric polarization of the ferroelectric film 103 canbe partially switched. In this way, more than two polarization statescan be stored in the layer stack 109, representing multiple binary logicstates. Thus, within one memory cell structure more than one bit can bestored, i.e., two bits with four polarization states or three bits witheight polarization states. For example the MFS structure 100 can be usedto form a FeFET, where the channel conductivity in the FeFET dependsupon the dipole orientation of the ferroelectric material oxide layer103 of the layer stack 109 of the MFS structure 100. In anotherembodiment, the MFM structure 110 can be utilized to form a capacitordielectric of a 1T-1C FeRAM. Here, the dipole orientation of theferroelectric material oxide layer 103 in the layer stack 109 determinesthe transient current and voltage level on the bit line during readoperation (the bit line is connected to the capacitor structure).

The various layers 103, 105, 106, 108 of the structures 100 and 110depicted in FIGS. 1a and 1b can be patterned at any time after formationof the layers, either before or after the annealing process tocrystallize the ferroelectric material oxide layer 103. The patterningof these layers may be carried out by an etch process using an etch mask(e.g., a hard mask), where the patterning of the layers can be carriedout based upon the intended use of these layers. For example, the layersmay be patterned to define at least part of a gate stack of a 1T FeFETor these layers may be patterned to define a capacitor dielectric of1T-1C FeRAM. Any suitable spacer structures may also be formed (e.g.,utilizing any suitable deposition technique, such as any of thedeposition techniques previously described herein) after patterning thelayers. Any other further processing of the carrier or support structureto integrate other components in relation to the support structure canbe implemented before, after or together with the formation of the MFSstructure 100 or MFM structure 110. For example, source/drain regions102 of the ferroelectric memory cells may be formed before, after ortogether with the ferroelectric layer.

It is to be understood that the layer stack 109 can comprise a singleferroelectric material oxide layer, or in another embodiment cancomprise a multitude of ferroelectric material oxide layers, or inanother embodiment can comprise additional conductive or isolatinginterfacial layers, separating the individual ferroelectric materialoxide layers.

With reference to FIGS. 2a and 2b , in one example the layer stack 109comprises a multi-layer stack of at least two ferroelectric materialoxide layers 203 and 204. The combination of the first and secondferroelectric material oxide layers 203 and 204 defines a combinedferroelectric layer stack 109. The different ferroelectric materialoxide layers can be separated by an interfacial layer 207, which cancomprise an insulating or a conducting material. It is to be understoodthat the ferroelectric layer stack 109 can comprise a multitude offerroelectric material oxide layers.

The ferroelectric layers 203 and 204 comprise different coercivevoltages, originating for example, but not limited to, from a differentcrystal orientation of the grains inside the ferroelectric materialoxide layers, a different internal stress, a different surface energydue to different grain sizes and different film thickness or differentdoping species and doping concentration. The ferroelectric domains 203 aand 204 a within each of the layers 203 and 204, respectively, canextend over the complete layer, or several similar domains can be formedwithin each of the layers besides each other. By applying a voltagelarger than the minimum coercive voltage but smaller than the maximumcoercive voltage of the multitude of ferroelectric material oxide layersin the combined ferroelectric layer stack 109, a first portion offerroelectric domains out of the plurality of all ferroelectric domainscan be switched. In that way different polarization states can bewritten into the combined layer stack 109, which represent multiplelogic bits.

For example, in one embodiment, the layer stack 109 comprises twoferroelectric material oxide layers 203 and 204 with different coercivevoltages. Therefore by combination of the two possible polarizationstates of the two ferroelectric material oxide layers 203 and 204 fourcombined polarization states of the layer stack 109 can be attained. Inreference to FIG. 2a in one embodiment layer stack 109 represents thegate oxide of a FeFET, where the four different polarization states ofthe layer stack 109 result in four different threshold voltages of thememory cell transistor 200, which can be sensed by a suitable sensingcircuit connected to the source, drain and gate electrodes. In referenceto FIG. 2b in another embodiment layer stack 109 represents thedielectric of a storage capacitor, where the four different polarizationstates of the layer stack 109 result in four different polarizationcurrents, which result from applying a read pulse to memory cellcapacitor 210, which can be sensed by a suitable sensing circuitconnected to the capacitor electrodes.

The ferroelectric material oxide layers 203 and 204 can be formed as waspreviously described in relation to the ferroelectric material oxidelayer 103. The ferroelectric material oxide layers 203 and 204 can beformed to have a thickness within the same ranges as previously notedfor the first ferroelectric material oxide layer 103. For example, eachof the first and second ferroelectric material oxide layers 203 and 204can have the same or substantially similar thicknesses. Alternativelythe first and second ferroelectric material oxide layers 203 and 204 canhave different thicknesses. The thickness of the combined ferroelectricmaterial oxide layer 109 can be within the range from about 500 nm toabout 2 nm, or from about 50 nm to about 1 nm. In addition, theferroelectric material oxide layers 203 and 204 can be formed toinclude, in addition to the ferroelectric material, dopants or furtheradditives that may support the crystallization of the layers 203 and 204into a state having ferroelectric properties. The dopant species anddopant concentration as well as the method of introducing the dopantinto the ferroelectric material oxide layers can be different for thetwo layers 203 and 204.

An interfacial layer 207 can be formed as conductive or isolating layeras was previously described in relation to the conductive layer 106. Itis to be understood, that the material of the interfacial layer can bechosen such that the work-function of the respective material influencesthe coercive voltage of the adjacent ferroelectric material oxide layersin a manner that induces a difference in the coercive voltage of theadjacent ferroelectric material oxide layers 203 and 204.

After the formation of the ferroelectric layers 203 and 204, anannealing step is carried out at one or more suitable temperatures andfor one or more suitable time periods to achieve a suitable amount ofcrystallization for the ferroelectric material within the ferroelectricmaterial oxide layers 203 and 204. The annealing step can be split upinto an annealing step which is performed first after formingferroelectric material oxide layers 203 and a second step which isperformed first after forming ferroelectric material oxide layers 203.

With reference to FIGS. 3a and 3b in another embodiment theferroelectric layer stack 109 is replaced by a multi-layer stack of atleast two ferroelectric material oxide layers 303 and 304. Theferroelectric material oxide layers are separated by a conductiveelectrode layer 307. The combination of the first and secondferroelectric material oxide layers 303 and 304 together with theconductive interlayer 307 defines a combined layer stack 109. It is tobe understood that the ferroelectric layer stack 109 can include amultitude of ferroelectric material oxide layers separated by conductivelayers.

The ferroelectric layers 303 and 304 comprise the same coercive voltagesor coercive voltages that differ by not more than 10% or not more than30% or not more than 70% or not more than 200% from each other.

The ferroelectric material oxide layers 303 and 304 can be formed as waspreviously described in relation to the ferroelectric material oxidelayer 103. The ferroelectric material oxide layers 303 and 304 can beformed to have a thickness within the same ranges as previously notedfor the first ferroelectric material oxide layer 103. For example, eachof the first and second ferroelectric material oxide layers 303 and 304can have the same or substantially similar thicknesses. Alternativelythe first and second ferroelectric material oxide layers 303 and 304 canhave different thicknesses. The thickness of the combined ferroelectriclayer stack 109 can be within the range from about 500 nm to about 2 nm,or from about 50 nm to about 2 nm. In addition, the ferroelectricmaterial oxide layers 303 and 304 can be formed to include, in additionto the ferroelectric material, dopants or further additives that maysupport the crystallization of the layers 303 and 304 into a statehaving ferroelectric properties. The dopant species and dopantconcentration as well as the method of introducing the dopant into theferroelectric material oxide layers can be equal or different for thetwo layers 303 and 304.

The conductive material layer 307 can be formed as was previouslydescribed in relation to the conductive layer 106. It is to beunderstood, that the material of the conductive layer can be chosen suchthat the work-function of the respective material influences thecoercive voltage of the adjacent ferroelectric material oxide layers ina manner that induces a difference in the coercive voltage of theadjacent ferroelectric material oxide layers 303 and 304.

After the formation of the ferroelectric layers 303 and 304, anannealing step is carried out at one or more suitable temperatures andfor one or more suitable time periods to achieve a suitable amount ofcrystallization for the ferroelectric material within the ferroelectricmaterial oxide layers 303 and 304. The annealing step can be split upinto an annealing step performed first after forming ferroelectricmaterial oxide layers 303 and a second step performed first afterforming ferroelectric material oxide layers 304. The ferroelectricdomains 303 a and 304 a within each of the layers 303 and 304,respectively, can extend over the complete layer, or several similardomains can be formed within each of the layers besides each other.

The ferroelectric material oxide layers 303 and 304 and the conductivelayer 307 can be patterned at any time after formation of the layers,either before or after the annealing process to crystallize theferroelectric material oxide layers 303 and 304. The patterning of theselayers can be carried out by an etch process using an etch mask (e.g., ahard mask), where the patterning of the layers can be carried out basedupon the intended use of these layers. For example, the layers may bepatterned to define at least part of a gate stack of a 1T FeFET or theselayers may be patterned to define a capacitor dielectric of 1T-1C FeRAM.Any suitable spacer structures may also be formed (e.g., utilizing anysuitable deposition technique, such as any of the deposition techniquespreviously described herein) during or after patterning of the layers.

Any other further processing of the carrier or support structure tointegrate other components in relation to the support structure can beimplemented before, after or together with the formation of the MFSstructure 300 or MFM structure 310. For example, source/drain regions102 of the ferroelectric memory cells may be formed before, after ortogether with the ferroelectric layer.

In one example of one embodiment, with reference to FIG. 3c , theferroelectric material oxide layers 303 and 304 together with theconductive interlayer 307 form a capacitive voltage divider, whichcomprises two capacitors C1 and C2. C1 is formed by the electrodes 106and 307 and ferroelectric material oxide layer 304, whereas C2 is formedby the electrodes 108 and 307 and ferroelectric material oxide layer303. The ferroelectric material oxide layers 303 and 304 can bepatterned such that the capacitances of two in series connectedcapacitors C1 and C2 differ due to a different area of the capacitors,respectively. By applying a voltage V to the combined layer stack 109,that is, applying a voltage between the upper electrode 106 and lowerelectrode 308, due to the capacitive voltage divider a larger voltagedrop is reached over the smaller capacitor. For example, for the case ofC1<C2, the voltage at C1, which is VC1=V (C1+C2)/C1, is larger than thevoltage drop at C2, which is VC2=V (C1+C2)/C2. Therefore in case ofequal coercive voltages of ferroelectric material oxide layers 303 and304 the polarization reversal will occur first in capacitor C1 that isin the ferroelectric material oxide layer 304. Upon further increase ofthe applied voltage V, also the coercive voltage of C2 will reached,which yields polarization reversal in C2, that is, in the ferroelectricmaterial oxide layer 303. In this manner, by combination of the twopossible polarization states of the two ferroelectric material oxidelayers 303 and 304, four combined polarization states of the layer stack109 can be attained. In reference to FIG. 3a , in one embodiment layerstack 109 represents the gate oxide of a FeFET, where the four differentpolarization states of the layer stack 109 result in four differentthreshold voltages of the memory cell transistor 300, which can besensed by a suitable sensing circuit connected to the source, drain andgate electrodes, as is depicted exemplarily in FIG. 1c . In reference toFIG. 3b , in another embodiment layer stack 109 represents thedielectric of a storage capacitor, where the four different polarizationstates of the layer stack 109 result in four different polarizationcurrents, which result from applying a read pulse to memory cellcapacitor 310, which can be sensed by a suitable sensing circuitconnected to the capacitor electrodes, as is depicted exemplarily inFIG. 1 d.

In one further embodiment with reference to FIG. 4, a ferroelectricmemory device 40 comprises a ferroelectric memory cell structure 400,comprising a ferroelectric layer stack 109, comprising a ferroelectricmaterial oxide layer 103, which represents the gate oxide of a FeFETmemory cell 400. The memory cell comprises two electrically coupled gateconductors 106 a and 106 b, which are patterned such that the area ofgate conductor 106 a is larger than the area of gate conductor 106 b.The ferroelectric material oxide layer 103 can be patterned self-alignedto the gate conductors 106 a and 106 b such that the area of theferroelectric domain structure 103 d is larger than ferroelectric domainstructure 103 e. Due to the different surface energy and internal stressof the differently sized grains, the coercive voltage of the coincidingferroelectric domains is different. In this manner, by applying avoltage V to both coupled gate electrodes within memory cell 400, thetwo patterned domains can be switched individually. As a result, bycombination of the two possible polarization states of the twoferroelectric domain structures 103 d and 103 e four combinedpolarization states of the layer 103 can be attained.

The source and drain regions 102 and 102 a can be formed as describedpreviously. In one embodiment, the source/drain region 102 a connectsthe two transistor channels that are formed by the patterning of thegate conductors 106 a and 106 b. In this manner, the memory cell areacan be reduced and no wiring via metallic contact structures andadditional metallization layers between the two transistor structures isrequired.

It is to be understood that the patterning of the ferroelectric materialoxide layers can be performed in a different way than by etch. Inanother embodiment, the ferroelectric material oxide layer is depositedinto a pre-structured trench, thus confining the formation of thedomains. In another example, the ferroelectric material oxide layer isdeposited self-aligned by self-aligned selective growth on top of apre-structured supporting structure 101, thus confining the formation ofthe domains. In this way, the targeted formation of ferroelectricdomains with targeted size can be realized in order to adjust theswitching properties of the single domains.

It is to be understood that the schematic cross-sectional view of theFeFET memory cell 100, 200 or 300 or MFM capacitor 110, 210 or 310 formsa part of an integrated circuit 10, 11, 21, 30, 31, respectively. Forexample, an integrated circuit 10 may comprise a plurality of FeFETmemory cells 100 arranged in the form of a ferroelectric memory cellarray. In another example, an integrated circuit 11 may comprise aplurality of ferroelectric capacitor memory cells 110, arranged in theform of a ferroelectric memory cell array. Additional circuits may alsobe formed in the support structure 101. By way of example, theseadditional circuits may include word line drive circuits, bit line drivecircuits, source line drive circuits, sense circuits, control circuits,and any other suitable circuits for the integrated circuit. In general,any semiconductor devices, e.g., diodes, bipolar transistors, diffusionresistors, silicon controlled rectifiers (SCR), and field effecttransistors (FET), may be formed within the support structure 101. Inaddition, a wiring area including a stack of conductive layers, e.g.metal layers, and intermediate dielectrics may be formed over portionsof the support structure 101, where the wiring area may be used tointerconnect semiconductor devices or circuit parts of the integratedcircuit.

It is further noted that the MFS structure 100, 200 or 300 can also beformed with any one or more suitable geometries including, withoutlimitation, planar or 3D geometry such as Trench MOSFET, FinFET, RCAT(“Recessed Channel Array Transistor”), TSNWFET (“Twin Silicon NanoWireField Effect Transistor”), PiFET (“Partially insulated Field EffectTransistor”), McFET (“Multi-channel Field Effect Transistor”).

With reference to FIG. 5, it is noted that the memory cell structures100, 200 or 300 can also be formed with any one or more suitablegeometries including, without limitation, 3D geometries such as memorycell structures 500 comprising pillar transistor channels 501,comprising a surrounding ferroelectric layer or a combined stack offerroelectric layers 503, comprising properties as described earlier forferroelectric material oxide layers 103, 203, 204, 303, 304, interfacelayers 105, 207 and electrode layers 307. Further, memory cell structure500 may comprise a multitude of gate electrode layers 506 which areisolated by isolation layers 502. Memory cell structure 500 can form avertical NAND-string of FeFET cell transistors.

It is further noted that the MFM structure 110, 210 or 310 can also beformed with any one or more suitable geometries including, withoutlimitation, planar or 3D geometry such as Trench capacitors, stackedcapacitors or cup-capacitors.

While the example embodiments described herein are in relation to 1Tferroelectric memory cells (FeFETs) and 1T-1C ferroelectric memorycells, the present invention is not limited to such memory cells butinstead can be applied to any suitable type of ferroelectric memory cell(e.g., 2T-2C memory cells).

Since the presence of the ferroelectric properties is necessarilyaccompanied by the presence of piezoelectric properties a ferroelectricmemory cell or a piezo element can be formed by using a MFM capacitor.Applying a certain voltage to the device will cause a piezoelectricexpansion of the device with can be used for different applicationswhere piezo-elements are included into a device to cause atransformation of an input signal (mainly an electrical signal) intomotion or to prevent a motion. The main advantage of a HfO₂ based piezoelement is that the material is lead free.

The embodiments described herein enhance the performance of aferroelectric memory cell and increases the memory bit densityassociated with the memory cell. In particular, multiple polarizationstates of the layer stack 109 enable the storage of multiple bits withinone memory cell. Thus the overall manufacturing costs per bit can bereduces.

As one example of other embodiments described herein, a method forprogramming a ferroelectric memory cell comprises a general voltagescheme for read and write operations and for the idle state for theferroelectric memory structure, depicted in FIG. 1a , FIG. 2a , FIG. 3aand FIG. 4, comprising voltage combinations as indicated in FIG. 6a .V_(G), V_(D), V_(S) and V_(B) represent voltage levels applied to thegate, drain, source and bulk terminals, respectively, as indicated inFIG. 1c , respectively. In the idle state, all terminals are grounded.Read operation comprises applying a read gate V_(G,r) and drain V_(D,r)voltages with bulk and source terminals grounded and sensing theresulting drain current. This current could assume a multitude ofdiscrete levels at a fixed sensing V_(G,r) and V_(D,r) voltages andwould correspond to a multitude of distinct stored polarization stateswithin the storage layer.

Referring to FIG. 1a and FIG. 4, a write operation of a ferroelectricmemory cell, comprising a voltage pulse which is applied to theferroelectric layer stack 109 of the memory cell to change the directionof the electric polarization, herein indicated as polarization, of aportion or of the totality of ferroelectric domains within theferroelectric material. For simplicity, structure in FIG. 1a will betaken as an example. The structure depicted in FIG. 1a can represent aNMOS FeFET. By first applying a negative voltage to the gate electrode106, such that it is higher in amplitude than the highest negativecoercive voltage |−U_(C,max)| in the storage layer, all ferroelectricdomains will have the polarization switched in one direction, developingthe highest possible threshold voltage of the NMOS transistor.Successively, applying positive voltage levels to the gate electrode 106greater than coercive voltage values corresponding to a portion of butnot to the totality of ferroelectric domains in the storage layer, onlythis portion of ferroelectric domains will be switched to the oppositepolarization. In this way, different intermediate threshold voltagelevels corresponding to the applied gate voltage amplitude, andtherefore to the quantity of the switched ferroelectric domains could becreated.

Referring to memory cell structures depicted in FIG. 2a and FIG. 3a , awrite operation comprises a voltage combinations indicated in FIG. 6a .It further comprises grounding drain, source and bulk terminals andapplying suitable combination of gate voltage levels V_(PP), V_(P),−V_(EE), and −V_(E) depending on the polarization state that is to bereached. It is to be noted that these voltages have to be tuned takinginto consideration the voltage divider within the gate stack.

Another example of a further embodiment described herein comprises apossible operating scheme, which is illustrated by the allowedtransitions between different intermediate states of the cell as isrepresented in FIG. 6b . This scheme applies for the ferroelectricmemory cells as exemplarily depicted in FIG. 2a and FIG. 3a . Forsimplicity, FIG. 2a will be taken as an example. The two arrowscorrespond to the polarization direction of the ferroelectric materialoxide layers 203 and 204. State ‘11’ and State ‘00’ correspond to thesituation where layers 203 and 204 have mutually parallel polarizationdirections and lead to two extreme threshold voltage levels of theFeFET. State ‘10’ and State ‘01’ correspond to the situation where 203and 204 have mutually antiparallel polarization directions and representtwo intermediate threshold voltage levels. State ‘11’ can be reached byapplying a positive gate voltage V_(PP) producing a voltage drop ineither 203 layer or 204 layer larger than the highest positive coercivevoltage +U_(C,max) of these two layers, independently of the previouslystored state. State ‘11’ can be reached by applying a negative gatevoltage −V_(EE) producing a voltage drop in either 203 layer or 204layer larger than the highest negative coercive voltage |−U_(C,max)| ofthese two layers, independently of the previously stored state. State‘10’ can be reached only starting from the already written State ‘11’and applying a negative pulse −V_(E), where V_(E)<V_(EE), in order toswitch the polarization of only one layer having a lower coercivevoltage. In this example embodiment state ‘01’ can be reached onlystarting from the already written State ‘00’ and applying a positivepulse V_(P), where V_(P)<V_(PP), in order to switch the polarization ofonly one layer having a lower coercive voltage. In this example thedirect state transitions from ‘11’ to ‘01’, from ‘00’ to ‘10’, as wellas from ‘10’ to ‘01’ and vice versa, are not feasible.

It is to be understood that present invention is not limited to theexemplarily described operation scheme and that the state-diagram can bedifferent, e.g. when implementing more than two ferroelectric materialoxide layers into the layer stack 109, or if the coercive voltage forprogram and erase operation exhibit different absolute values, which canoriginate from different work-functions of the electrodes 106, 101, 108or conductive interlayers 307.

Ferroelectric capacitor structures illustrated in FIGS. 1b, 2b and 3bare intended to be integrated in 1T-1C multi-level FeRAM memory cell, asdepicted in FIG. 1d . Here, a conventional NFET acting as an accesstransistor is adopted to perform write and read operations: in bothoperations the word-line (WL) is asserted, which means a high voltageVWL is applied in order to induce a conductive channel connecting thesource-line (SL) and the ferroelectric capacitor. In the idle state, theWL is not asserted, leaving the access transistor in its OFF state. In asimilar manner as previously described for the 1T writing patterns,taking into account the voltage divider for structures in FIG. 2b and inFIG. 3b , the 1T-1C cell can have the same voltage scheme, with theexception that the positive V_(PP) and V_(P) voltages are applied to SLwhile BL is grounded, whereas the positive V_(EE) and V_(E) voltages areapplied to BL while SL is grounded. In this way, negative voltages canbe completely avoided, as known for the standard 1-bit FeRAMarchitecture. A cell can be read by floating the SL and applying apositive voltage (V_(DD)) to the BL while asserting the WL. The distinctstored levels will induce then distinct displacement current signals tobe sensed at SL. Since this operation is destructive, the data must bewritten back into the cell after each read (data restore).

Although specific embodiments have been illustrated and describedherein, it will be appreciated by those of ordinary skilled in the artthat a variety of alternate and/or equivalent implementations may besubstituted for the specific embodiments shown and described withoutdeparting from the scope of the present invention. This application isintended to cover any adaptions or variations of the specificembodiments discussed herein. Therefore, it is intended that thisinvention be limited only by the claims and the equivalents thereof.

What is claimed is:
 1. An integrated circuit element comprising: a firstelectrode; a second electrode; and a storage layer disposed between thefirst and the second electrodes, the storage layer comprising a materialpartially in a ferroelectric state and having two or more ferroelectricdomains corresponding to different grains in the material, each of theferroelectric domains within the material having different coercivefields such that voltages necessary to reach the coercive fields of thetwo or more ferroelectric domains are different.
 2. The integratedcircuit element of claim 1, wherein the material of the storage layercomprises a polycrystalline film having a same thickness and compositionin each of the two or more ferroelectric domains.
 3. The integratedcircuit element of claim 1, wherein the two or more ferroelectricdomains differ in at least one of: crystal orientation; internal stress;grain size; and doping concentration.
 4. The integrated circuit elementof claim 1, wherein the voltages necessary to reach the coercive fieldsof two of the two or more ferroelectric domains differ by at least 50 mVfrom each other.
 5. The integrated circuit element of claim 1, where thetwo or more ferroelectric domains in the storage layer disposed betweenthe first and the second electrode are arranged on top of each other. 6.The integrated circuit element of claim 1, wherein the storage layerfurther comprises at least one interlayer disposed between the first andsecond electrodes.
 7. The integrated circuit element of claim 6, whereinthe at least one interlayer is conductive.
 8. The integrated circuitelement of claim 1, wherein the storage layer comprises one or more ofthe following additives: C, Si, Al, Ge, Sn, Sr, Mg, Ca, Sr, Ba, Ti, Zr,Hf, Gd, Sc, La or their oxides.
 9. The integrated circuit element ofclaim 8, wherein the concentration of at least one additive is withinthe range from about 0.05% to about 5%.
 10. The integrated circuitelement of claim 1, wherein the material of the storage layer comprises:hafnium oxide, zirconium oxide or a solid solution of hafnium oxide andzirconium oxide.
 11. The integrated circuit element of claim 1, whereinthe thickness of the material of the storage layer partially in theferroelectric stage range from 2 to 15 nm.
 12. The integrated circuitelement of claim 1, wherein the first and second electrodes comprise oneor more of the following: Ti, TiN, TiSi, TiAlN, TaN, TaCN, TaSi, W, WSi,WN, Ru, RuO, Re, Pt, Ir, IrO, Ti, Ni, NiSi, Nb, Ga, GaN, C, Ge, Si, SiCor GeSi.
 13. The integrated circuit element of claim 1, wherein thefirst or second electrode and the storage layer form at least part of agate structure or a FeFET.
 14. The integrated circuit element of claim1, wherein the first or second electrode and the storage layer form atleast part of a capacitor.
 15. The integrated circuit element of claim1, wherein the integrated circuit element is arranged in a plurality ofintegrated circuit elements.